The invention is based on a metering arrangement and a metering method of the generic type.
In auto-ignition internal combustion engines, an SCR catalytic converter is frequently provided in the exhaust gas system in order to comply with environmental regulations. So that the SCR catalytic converter can convert the NOx compounds contained in the exhaust gas into water and atmospheric nitrogen, an exhaust-gas aftertreatment medium, for example a liquid urea/water solution which serves as a reducing agent, is injected into the exhaust gas section upstream of the SCR catalytic converter. For this purpose, for example a metering system comprising a reducing agent tank, a reducing agent pump and a metering module which operates similarly to the injector of a fuel injection system are used. The pump and assigned components are also referred to as a feed module.
The objective of the feed module or of the pump is to suck urea/water solution out of the tank and build up sufficient pressure on the pressure side so that the liquid urea/water solution is finely atomized as soon as the metering module is opened in a way which is controlled according to requirements and feeds the exhaust-gas aftertreatment medium in particular to the exhaust gas system downstream of the engine. The injector is therefore connected, like the feed module, to a control device of the internal combustion engine and is opened and closed thereby in accordance with requirements. The same also applies to the operation of the feed pump. Since urea/water solution has the property of freezing at low temperatures and at the same time increasing its volume by approximately 11%, measures have to be taken to prevent damage to the metering system by freezing urea/water solution.
For this purpose, EP1812144 discloses ventilating lines which conduct urea/water solution. For this purpose, the pump is designed with a reversible feed direction and a valve for reversing the feed direction of the pump is provided. Said document describes, as does also DE102011081628 published after the priority date of said document, that optionally a second pump can be provided in order to pump the urea solution out of the region of the feed line.
From DE102009029408 it is also known to integrate a 4/2-way valve into the metering system. In a first switched position of the 4/2-way valve the pump feeds reducing agent from the tank to the metering module. If the internal combustion engine is to be switched off, the 4/2-way valve is placed in the second switched position, with the result that the pump of the feed module feeds liquid reducing agent from the metering module into the tank and as a result ventilates parts of the metering system. This requires the metering module to be opened and air or exhaust gas to be able to flow on into the metering system from the exhaust gas section.
As a result of the partial ventilation of the metering system a compressible air bubble is produced, with the result that if the remaining reducing agent in the metering system is frozen, the pressure of the ice which results therefrom is so low that no damage can occur to the metering system. Such a 4/2-way valve is, however susceptible to faults and expensive.